cmake: remove case mismatch detection on build mode
cmake: remove case mismatch detection on build mode

See discussion here for context:
https://github.com/ziglang/zig/commit/c6df5deb3450e0d8a2ba449c34a0bd195fbce8ec#comments

Michael - I appreciate what you did here, making the configure script
work better for people in practice. When it was checking the build type
against a whitelist, I think it was worth it. However, now that we are
supporting systems which use non-standard cmake build modes, I don't
think this case-mismatch detection thing is worth it. It's starting to
get to the point where it's a lot of complication for very little
benefit. Besides, cmake is not case sensitive. If we support
non-standard build modes, then we would need to support a hypothetical
build mode of `release` (lower case).

So let's just remove this and rely on people to use the build system
correctly (like they will have to do when building any cmake project
from source).

raulgrell/zig,raulgrell/zig,raulgrell/zig,zig-lang/zig,raulgrell/zig,zig-lang/zig,raulgrell/zig,andrewrk/zig,andrewrk/zig,zig-lang/zig